1. Field of the Invention
The present invention generally relates to a static dissipative resin composition, and more particularly, to a resin composition having the resistivity (surface resistivity 1.0.times.10.sup.5 -1.0.times.10.sup.12 .OMEGA./.quadrature.) defined as the static dissipative property according to the U.S. standard ANSI/EIA-541-1988, or a resin composition having the resistivity (surface resistivity 1.0.times.10.sup.5 -1.0.times.10.sup.9 .OMEGA./.quadrature.) defined as the static dissipative property according to the U.S. standard DOD-HOBK-263.
The resin composition of the present invention is used in the form of pellets or powders as a resin compound for injection molding or extrusion molding, etc. and is also offered as a final molded product like a part, a plate, a sheet, a film, etc. Moreover, the resin composition may be offerred as a paint, coating paste or putty or a coated film.
For examples of the aforementioned molded final product, there are wall or floor materials (floor tile, floor surfacing and the like), antistatic packaging products for semiconductors (e.g., IC trays, IC magazines, IC carrier tapes, boxes, containers, cabinets, substrate holders, printed board stands and the like), and the other antistatic products, earthing products and sliding parts or the like in the OA, AV, FA fields such as computers, copying machines, facsimiles, printers, VTRs, video cassette tapes, compact disk recorders, etc. More specifically, tape guides and guide rollers for the tapes of the AV devices can be manufactured of the resin composition, and such sliding parts as bearings and gears are formed of the resin composition of the present invention. Likewise, the resin composition is useful not only for the components of liquid crystal displays, particularly, TFT liquid crystal displays, but for the supporting or holding members in the manufacturing process of the displays. The resin composition may be employed to coat the floor and equipments in a clean room.
2. Description of the Prior Art
Resin has been widely employed as a material having a superior insulation property (for instance, 10.sup.15 .OMEGA./.quadrature. or higher).
In recent years, however, the static electricity generated from the insulative resin has been noticed as a problem of the semiconductors, OA or FA devices, and accordingly a resinous material with static dissipative properties (10.sup.5 -10.sup.12 or 10.sup.5 -10.sup.9 .OMEGA./.quadrature.) has attracted people's enthusiastic attention. Although a highly conductive resin (smaller than 10.sup.5 .OMEGA./.quadrature.) is effective to some degree so long as the prevention of static build-up and static electricity are concerned, it is an imperfect resin since, because of the highly conductive properties thereof (smaller than 10.sup.5 .OMEGA./.quadrature.), a discharge spark is brought about by the static electricity or a short-circuiting occurs with the general use electricity.
The static dissipative material has a proper resistance by itself, and therefore the static electricity, if it is generated, can be easily and promptly let outside, without accompanying discharge spark. In addition, the static dissipative material has little possibilities for an electric shock or short-circuiting in a general electric circuit, etc. In other words, the static dissipative material is regarded as "statically conductive and electrically insulative".
Therefore, various developments have targeted a superior static dissipative material, but a fully satisfactory result has not been achieved yet.
For instance, conventionally, an organic and ionic antistatic agent has been mixed into ABS resin or the like. But this resinous material is strongly dependent on humidity, and it hardly functions in the highly dried condition. Even worse, the antistatic agent may ooze out of the surface of the resin with time, and only a relatively high resistance (e.g., (10.sup.10 -10.sup.12 .OMEGA./.quadrature.) is achieved by this resinous material. The resinous material of this kind is therefore instable. Moreover, this method is applicable to a limited kind of resins.
On the other hand, an inorganic conductive filler having more stable characteristics than the above-described antistatic agent has been mixed in some of the resinous materials.
In general, the following has been known about the resinous material of this type. When the conductive filler is mixed into a resin thereby to turn the resin conductive, the phenomenon in this case is often explained by the "Percolation Theory". The relation between the mixing amount (wt. %) of the filler and resistance of the resinous material is schematically represented in a diagram of FIG. 1. More specifically, as the conductive filler is mixed, the resinous material is initially in the originally insulative state (area (I)). When the mixing amount exceeds a certain level, a slight conductivity appears immediately (although the conductivity is instable) (area (II)). Even if the mixing amount of the filler is increased over the certain level, the conductivity Rs is not changed so much, that is, it enters a stable area (III). Therefore, a stable conductive resinous composition is designed in the area (III).
Meanwhile, a stable point Rso of the conductivity of the resinous composition is greatly dependent on the conductivity of the conductive filler. Rso becomes not higher than 10.sup.-1 .OMEGA./.quadrature. in the case of a metallic conductive filler; and also becomes (10.sup.0 -10.sup.4 .OMEGA./.quadrature. in the case of carbon filler.
As is apparent from FIG. 1, although it is not impossible to obtain the resinous material having the static dissipative level which exists between insulation level and Rso, in the middle of the insulation properties if the resinous material is designed in the area (II), the resultant resinous material is instable in conductivity with poor reproducibility.
As such, in order to obtain especially a static dissipative resinous composition, such a conductive filler that has the conductivity to bring the stable point Rso in the static dissipative level is desirable and strongly recommended. However, as mentioned before, the stable point Rso is too low in the case of conventional inorganic conductive fillers (either metallic or carbon filler) to be used for the static dissipative resin composition. If those conductive fillers are nevertheless used and the static dissipative resin compositions are designed in the area (II), the resin compositions are turned instable in conductivity and poor in reproducibility.
In the meantime, Japanese Patent Laid-Open Publication No. 1-225663 (225663/1989) reveals a resin composition with high conductivity (the 10.OMEGA..cm mark) having highly conductive zinc oxide whiskers mixed into a resin. This prior art is also disadvantageous in its extraordinary low Rso and is unable to achieve the stable static dissipative level, similar to the above-described resinous composition having the inorganic conductive filler mixed therein.